1. Field of the Invention
The present invention relates to a method of fabricating a martensitic stainless steel including the following heat treatment steps:
1) heating the steel to a temperature higher than the austenizing temperature TAUS of the steel, then quenching the steel until the hottest portion of the steel is at a temperature less than or equal to a maximum temperature Tmax, and greater than or equal to a minimum temperature Tmin, the rate of cooling being sufficiently fast for the austenite not to transform into a ferrito-perlitic structure;
2) performing a first anneal on the steel followed by cooling until the hottest portion of the steel is at a temperature less than or equal to said maximum temperature Tmax and greater than or equal to said minimum temperature Tmin; and
3) performing a second anneal of the steel followed by cooling to ambient temperature TA.
Ambient temperature is equal to the temperature of the premises where the method is performed.
In the present invention, composition percentages are given as percentages by weight, unless specified otherwise.
2. Description of the Related Art
A martensitic stainless steel is a steel in which the chromium content is greater than 10.5% and in which the structure is essentially martensitic (i.e. the quantity of alphagenic elements is sufficiently high compared with the quantity of gammagenic elements—see the explanations given below).
The starting material is a semi-finished product of arbitrary shape, e.g. in the form of a billet or a bar of the steel.
The semi-finished product is then precut into sub-elements that are shaped (e.g. by forging or rolling) in order to give them a shape close to their final shape. Each sub-element thus becomes a workpiece (also referred to as a “blank”) with extra thicknesses compared with the final dimensions it is to have in use.
The blank with extra thicknesses is subsequently to be machined in order to give it its final shape (finished part).
When finished parts are to possess a high degree of dimensional accuracy (e.g. in aviation), the blanks need to be subjected to heat treatment (quality heat treatment) prior to machining. This quality heat treatment cannot be performed after the machining since that would lead to changes in dimensions that are difficult to predict for parts of complex shape.
This quality heat treatment enables the properties of the steel workpiece to be adjusted very finely by performing metallurgical transformations, comprising six major stages:
A) austenization, i.e. heating to above the temperature at which the microstructure of the steel is transformed into austenite (austenitic temperature TAUS);
B) followed by quenching;
C) followed by a first annealing treatment;
D) followed by cooling;
E) followed by a second annealing treatment; and
F) followed by cooling.
The purpose of stage A) is to homogenize the microstructure within the workpiece and to put back into solution particles that are soluble at that temperature by recrystallization.
Stage B) is for performing a first maximum transformation of austenite into martensite within the steel workpiece. Nevertheless, transformations of the martensitic microstructure do not take place simultaneously at all points within the workpiece, but gradually starting from its surface and going to its core. The changes in crystallographic volume that accompany such transformations therefore lead to internal stresses and, at the end of quenching (because of the low temperatures that are then reached), they limit the extent to which the stresses can be relaxed. The second purpose is to minimize the risk of quenching cracks appearing as a result of residual stresses being released in the steel while it is in a martensitic state having low toughness. In order to achieve these two contradictory purposes, it is common practice to begin by heating the workpiece once more in an anneal treatment (stage C)) once its hottest portion has cooled to a temperature lying in a range defined by a maximum temperature Tmax and a minimum temperature Tmin for avoiding cracking. The temperature Tmax is substantially equal to the nominal temperature MF for the end of martensitic transformation of the steel, i.e. 150° C. to 200° C. for a martensitic stainless steel. The temperature Tmin lies in the range 20° C. to 28° C. depending on chemical composition. There then remains a residual austenite content in the steel that it has not been possible to transform.
Stage C)—first annealing treatment—this quality heat treatment has the purpose firstly of transforming the fresh martensite into annealed martensite (more stable and tougher) and also of destabilizing the residual austenite from the earlier stages.
Stage D)—cooling the first anneal—this quality heat treatment is intended to transform the residual austenite into martensite. The hottest portion of the workpiece must also be cooled to a temperature in the temperature range [Tmax, Tmin].
Stage E)—second annealing treatment—this quality heat treatment is intended to transform the new fresh martensite into annealed martensite (more stable and tougher), seeking to achieve a better compromise in the mechanical properties of the steel.
Stage F)—cooling the second anneal—this quality heat treatment returns the blank to ambient temperature.
In spite of this quality heat treatment, while workpieces are being machined, it is found at present that there is a large amount of dispersion in the machineability of batches of workpieces made of a steel that is the result of such a fabrication method. This can lead to large variations in the amount of wear of machining inserts and in large variations in the levels of power that the machine tool needs to deliver in order to be able to machine such steel workpieces. Consequently, the consumption of machining inserts is too high, too greatly dispersed, and unpredictable, thereby giving rise to reduced rates of throughput when machining batches of workpieces, and also to dispersion in the resulting surface states, sometimes leading to workpieces with machined surface states of poorer quality.